Organic molecular probes are designed to selectively associate with specific types of biomembranes. The probes are comprised of two structural components: a membrane targeting ligand and a photoactive reporter group. The primary membrane targets are the surfaces of human cancer cells, dying cancer cells, and bacteria, with applications ranging from in vivo optical imaging of tumors to optical imaging of bacteria to antibiotic photodynamic therapy. The major aims are: 1. Develop squaraine-rotaxanes as a new family of fluorescent near-IR organic dyes with unprecedented brightness and high stability. The dyes will be converted into probes for various types of optical imaging applications including imaging of growing or dying tumors in murine xenograft models. The eventual objective is to detect tumors in living animals or monitor the effectiveness of anticancer therapy. 2. Synthetic zinc dipicolylamine (Zn-DPA) affinity ligands will be attached to various fluorescent scaffolds to produce new optical imaging probes for bacteria. Preliminary results show that implanted murine bacterial infections can be imaged with probes that emit near-IR fluorescence;the first example of targeted optical imaging of bacteria in a living animal. The scope of this novel imaging method will be determined, including the selectivity and sensitivity for various strains of bacteria. Additional studies will develop highly sensitive fluorescent methods for detecting bacterial contamination in mixed media, especially biomedical samples and food products. 3. Bifunctional probes with Zn-DPA affinity ligands attached to a photosensitizer will be evaluated for utility in antibiotic photodynamic therapy. Since the Zn-DPA affinity ligands target ubiquitous polar lipids in the bacterial cell membrane, these bifunctional probes will be effective against most forms of drug resistant bacteria and prolonged treatment with these probes is unlikely to induce high levels of bacterial resistance. Relevance to Public Health: The fluorescent probe molecules produced by this research will be used to image cancer tumors and bacterial infection in living animals. The imaging technology will help researchers discover new therapies and eventually it will help physicians choose the most appropriate therapy for a specific patient. This latter application is an example of how optical imaging will contribute to the evolving concept of personalized medicine.